Epoxy resins exhibit high adhesion to various substrates. Cured products of epoxy resins have relatively excellent properties, such as heat resistance, chemical resistance, electric characteristics, and mechanical characteristics, and, are useful in a wide range of applications, including coatings, adhesives, and molding materials.
Conventional epoxy resin compositions are mostly of two-pack type in which an epoxy resin component is mixed with a curing agent or a curing accelerator on use. A two-pack type epoxy resin composition is characterized by curability at ambient or low temperature. Nevertheless, a two-pack epoxy resin composition is disadvantageous in that the two packs must be metered and mixed immediately before use and that the pot life of the composition is short and therefore has a limited use, for example, meeting difficulty in applying to automatic machinery. To eliminate these disadvantages, a one-pack type epoxy resin composition has been demanded.
In order to develop a one-pack type curing resin composition, a curing agent that does not react at room temperature but commences reaction to cure upon heating, i.e., a latent curing agent is necessary. Latent curing agents so far proposed include dicyandiamide, dibasic acid hydrazides, amine-boron trifluoride complex salts, guanamines, melamine, and imidazoles. They have, however, their own disadvantages. That is, mixtures of en epoxy resin with dicyandiamide, melamine, or guanamines are, while excellent in storage stability, problematic in that they need a high temperature (150° C. or higher) and a long time to cure. It is widely practiced to use a curing accelerator in combination to shorten the curing time, but this is accompanied by remarkable reduction in storage stability. On the other hand, compositions containing a dibasic acid dihydrazide or an imidazole cure at relatively low temperatures but have poor storage stability. While compositions containing an amine-boron trifluoride complex salt have high storage stability and cure in short times, they are poor in water resistance and corrosive to metals.
Patent Document 1 below proposes using an ionic liquid composed of, e.g., a combination of an ammonium- or phosphonium-based cation and a carboxylate anion as an epoxy resin curing agent. The results are still unsatisfactory.